Exceeding the stress limits of a typical composite ball bat, or other fiber-reinforced composite structure, may allow for an increase in bat performance, in terms of ball exit velocity. This performance increase occurs largely as a result of micro-crack accumulation in the ball bat's resin system, due to a combination of residual stress relief and repeated load application, which results in a slight increase in bat compliance. The amount of the performance increase is generally dependent upon the specific bat design and the materials used to construct the bat.
This performance increase, however, is asymptotic. In other words, as the number of impacts becomes very large, the change in micro-crack density reaches a constant value, such that there is no further performance increase from additional impacts. It is for this reason that a significant number of commercially available composite ball bats are designed to produce a ball exit velocity at least 2 to 4 mph below governing body (e.g., the Amateur Softball Association, or the “ASA”) approval limits. In other words, a tolerance of a 2 to 4 mph performance increase, as a result of micro-crack accumulation, is “built into” a typical bat design. In this manner, regardless of the age of the bat structure, the performance limit should not be exceeded under normal use conditions.
As a result of the awareness of this “bat break-in” performance advantage, methods of increased performance acceleration were sought by players trying to gain an increased advantage. These methods have included, but are not limited to, repeatedly hitting a bat against a tree, curb or fencepost, freezing a bat and hitting it with a bowling ball, and putting a bat in a vice and compressing it until the batter hears an audible “pop.” All of these techniques severely alter the bat barrel kinetics by breaking down the shear strength between the laminate plies, essentially increasing the number of composite walls present in the structure. The mechanism by which this is achieved is referred to as accelerated break-in (“ABI”).
These ABI methods generally do not accelerate micro-crack accumulation (i.e., the natural break-in (“NBI”) process), but instead target the weak interlaminar region of the composite structure, which leads to interlaminar fracture or delamination. Delamination is a mode of failure that causes composite layers within a structure to separate, resulting in significantly reduced mechanical toughness of the composite structure. The strength at which a composite structure fails by delamination is commonly referred to as its interlaminar shear strength.
Delamination typically provides significantly increased bat compliance, or increased “trampoline effect,” which may result in a ball bat that exceeds association performance limits. Because of this phenomenon, which is not readily detectable, governing bodies are considering enacting stricter compliance limits. These proposed limits could require a ball bat to initially perform well below acceptable association limits, in order to account for the potential performance increase resulting from delamination. As initially constructed, ball bats meeting these increased standards would typically perform poorly and have a bad “feel,” thus greatly reducing the desirability of the composite ball bats.